1. Field of the Invention
The present invention relates to the field of the manufacture of chopped thermoplastic yarns, in particular of glass yarns, and it relates more specifically to a system for the manufacture of such yarns.
2. Description of the Related Art
Numerous devices capable of carrying out such manufacturing operations are known. These systems generally comprise at least one bushing from which glass yarns are drawn and conveyed to a chopping device composed, for example, of a back-up roll or anvil which interacts with a chopping roll equipped with evenly distributed blades at its periphery. The chopper is positioned so as to come into contact under pressure with the circumferential surface of the back-up roll, thus defining a chopping region.
The French certificate of addition FR 2 075 019 illustrates a system of this type in which a bushing is used in combination with a chopping device.
In order to increase the production efficiency of plants for the manufacture of chopped yarns, solutions have been devised according to which several bushings feed a single chopping device. French patent FR 2 490 251 illustrates a solution of this type. These techniques exhibit a priori the advantage of having a single chopping machine used in combination with several bushings.
However, in use, these plants display several disadvantages: first, rapid wearing of the chopping rolls, this being because these rolls have to chop very large amounts of yarns. Production is ordinarily of the order of 12 tones of chopped yarns per day, so that the chopping rolls have to be replaced after 6 to 8 hours of operation. This arrangement exhibits the following disadvantages:                the loss of glass flowing from the bushings        thermal conditions for the bushings and duct which are disrupted.        
Furthermore, as several yarns feed a single chopper, the dispersion with regard to the glass delivery of each bushing makes it necessary to consider the mean of the deliveries for the calculation of the speed of a chopping device. That is to say, a high standard deviation with regard to the count, which is not satisfactory with regard to the quality of the finished product and which gives outputs which decline in proportion as the bushing departs from the standard.
In addition, the current tendency is to increase the number of filaments per strand. Each strand thus becomes increasingly difficult to chop. This results in uneven chopping with more or less fine offshoots and, consequently, in a nonhomogeneous density of the final product. An increase in the chopping pressure, which prematurely wears out the wheels and can result in melting phenomena, should also be noted.
Another important problem related to this concept concerns the splittings of the yarns. These splittings or “breakages” lead to human intervention to restart the yarn in the chopping machine. This incident is therefore particularly damaging to the production output as it requires a relatively long preparation time.
Furthermore, the broken end from a bushing can interact and result, by rolling up on a guiding member, in the breakage of the other bushings, with then the necessity to restart all the bushings. This human intervention indeed necessitates a very long period of interruption of the production since the operator then has to successively restart each of the yarns.